Add 'nix store optimise' manpage

src/nix/optimise-store.cc

@@ -13,14 +13,11 @@ struct CmdOptimiseStore : StoreCommand
         return "replace identical files in the store by hard links";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To optimise the Nix store:",
-                "nix store optimise"
-            },
-        };
+        return
+          #include "optimise-store.md"
+          ;
     }
 
     void run(ref<Store> store) override

src/nix/optimise-store.md

@@ -0,0 +1,23 @@
+R""(
+
+# Examples
+
+* Optimise the Nix store:
+
+  ```console
+  nix store optimise
+  ```
+
+# Description
+
+This command deduplicates the Nix store: it scans the store for
+regular files with identical contents, and replaces them with hard
+links to a single instance.
+
+Note that you can also set `auto-optimise-store` to `true` in
+`nix.conf` to perform this optimisation incrementally whenever a new
+path is added to the Nix store. To make this efficient, Nix maintains
+a content-addressed index of all the files in the Nix store in the
+directory `/nix/store/.links/`.
+
+)""